1. Field of the Invention
The present invention is directed to electric machines, and more particularly to stepper motors. Favorable embodiments of this invention are related to stepper motors which can be linearly advanced or retracted by application of stepper current, and which will automatically return to a zero position when current is cut off.
2. History of the Prior Art
There have been developed a number of electric rotating machines which can serve as stepper motors. Representative motors of this type are disclosed for example in U.S. Pat. Nos. 3,549,918 to Croymans et al., 3,633,055 to Ivan W. Maier, and 3,508,091 to R. J. Kavanaugh. The construction of these motors is quite similar to that of synchronous motors and, to an extent, the two types of motors are more or less interchangeable. Typically, these stepper motors include one or more coaxial stacked stator magnets with a cavity extending along a central axis, and a permanent magnet rotor disposed in the cavity. The stator magnets each include a toroidal winding and a stator yoke or case with axial teeth pointing in alternate directions to serve as salient stator poles. The teeth of each stator magnet are offset from the teeth of the other stator magnets to give the stepper motor the capability of being stepped in either rotational direction, i.e., clockwise or counterclockwise.
While these stepper motors act satisfactorily for imparting controlled rotational motion, they have not been adaptable to use as linear stepper motors to impart a controlled linear displacement.
Recently it has been desired to use electronic and electromechanical mechanisms for the automation of various functions formerly performed by purely mechanical means. For example, in the automotive industry, it has been desired to use an electronic system to control carburetor valves and fuel injection pumps. This would enable the fuel flow and other engine operating characteristics to be controlled under the command of a microprocessor and based upon various parameters such as engine speed, engine load, degree of accelerator actuation, etc. Thus, an electronic system can thereby achieve optimum engine performance at peak efficiency. A stepper motor could then be incorporated in the system to translate electrical information (in the form of pulses) into mechanical motion to control the carburetor or fuel injection system.
Unfortunately, no appropriate stepper motor has previously been available. In order to operate in the environment of an automotive internal combustion engine, not only must the motor enjoy long life in an atmosphere containing lubricants and fuel (gasoline or diesel fuel) at temperatures ranging from -40.degree. C. to +85.degree. C., it must also reliably deliver certain performance characteristics. In order to actuate the carburetor or fuel injector properly, the stepper motor must move in 0.002 to 0.004 inch steps and provide a linear thrust of about ten pounds. The motor must respond to each pulse and operate at a speed of up to 200 pulses per second. In addition, in the case of power loss (or switch-off of ignition) the linear stepper motor must retract to a withdrawn, or zero start position. Furthermore, the motors must be of a rather straightforward design so that the production costs thereof are kept reasonably low.